This invention pertains to a process for the preparation of aryl carboxylate esters. More specifically, this invention pertains to a process for preparing aryl carboxylate esters by the reaction of a phenol reactant with an carboxylic acid in the presence of trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAA).
Aryl carboxylate esters such as phenolsulfonate carboxylate esters are useful bleach activators (Allan H. Gilbert, Detergent Age, 1967, June, pages 18-20 and August, pages 30-33). Aryl carboxylate esters also are of commercial interest as components of liquid crystals and polyarylate liquid crystal polymers. A number of methods for synthesizing these aryl carboxylate esters are described in the literature. These known procedures, in general, require relatively harsh conditions and proceed slowly to completion. The synthesis of aryl carboxylate esters by boric acid catalysis is described by William W. Lowrance in Tetrahedron Letters, 1971, 37, 3453 and in U.S. Pat. No. 3,772,389. The preparation of carboxylate esters by the reaction of an alkanoic acid with a phenolsulfonate salt, e.g., sodium 4-phenolsulfonate (SPS) in the presence of boric acid as described in U.S. Pat. No. 4,478,754 requires many hours at temperatures greater than 180xc2x0 C. More active carboxylic acid derivatives such as acid chlorides and anhydrides react with SPS under milder conditions. These reactions are carried out either in a solvent or the carboxylic acid related to the desired ester product at temperatures of 80 to 200xc2x0 C. Esterification of phenolsulfonate salts using carboxylic acid anhydrides as the esterification agent is the preferred route for commercial scale synthesis although other technologies such as aryl carboxylate ester sulfonation described in the literature: Harold R. W. Ansink and Hans Cerfontain, Recl. Trav. Chim. Pays-Bas, 1992, 111, 215-21; U.S. Pat. No. 4,695,412.
U.S. Pat. No. 4,587,054 discloses the reaction of a C6-C18 carboxylic acid anhydride and substituted phenol at temperatures between 80-120xc2x0 C. using strong acid catalysis or at temperatures between 180-220xc2x0 C. using base catalysis. In the examples, the acid-catalyzed process is carried out at 90-100xc2x0 C. for four hours and the base-catalyzed process is carried out at 200xc2x0 C. for two hours. Similarly, U.S. Pat. Nos. 4,588,532 and 4,883,612 describe the reaction of a C7-C12 carboxylic acid anhydride in a polar aprotic solvent with SPS in the presence of a catalytic amount of sulfonic acid at temperatures xe2x80x9cin excess of about 100xc2x0 C.xe2x80x9d An example illustrates the operation of the process at 115 to 120xc2x0 C. for a period of six hours. U.S. Pat. Nos. 4,588,532 and 4,883,612 also disclose a base-catalyzed process also using a polar aprotic solvent xe2x80x9cin excess of 80xc2x0 C.xe2x80x9d An example describes a base-catalyzed experiment carried out at 90xc2x0 C. for three hours. U.S. Pat. No. 5,534,642 discloses the reaction of an amido-substituted carboxylic acid anhydride with a phenolsulfonate salt at 180xc2x0 C. for 3 hours.
A disadvantage of these known processes wherein carboxylic acid anhydrides are reacted with substituted phenols is that one equivalent of by-product carboxylic acid is produced for each equivalent of the desired aryl carboxylate ester. Thus, processes utilizing anhydrides must recycle the by-product carboxylic acid to be economically attractive. Because the carboxylic acids co-produced in commercial processes typically are high boiling, e.g., C6-C18 carboxylic acids, a simple evaporation of the by-product acid, even at reduced pressure, can require elevated temperatures and associated problems such as formation of color bodies. Likewise, a disadvantage of strong acid-catalyzed processes is the coincident catalysis of desulfonation of the phenolsulonate reactant leading to yield losses and darker colored products.
Procedures for the synthesis of phenolsulfonate alkanoate esters by transesterification, either by alcoholysis or acidolysis, have been published. For example, U.S. Pat. No. 4,537,724 discloses the alcoholysis of phenyl nonanoate with SPS to obtain sodium 4-(nonanoyloxy)benzenesulfonate in 83% yield after heating for four hours at 290-300xc2x0 C. Alternatively, European Patent Publication EP 105,672 discloses the acidolysis of C2-C3 alkanoyloxybenzene sulfonates with C6-C18 aliphatic carboxylic (alkanoate) acids, driven by the removal of the lower boiling C2-C3 acids. In an example, nonanoic acid reacts with acetyloxybenzene sulfonate, using sodium acetate as a catalyst, at 166-218xc2x0 C. over 3.5 hours. Similarly, U.S. Pat. No. 5,534,642 discloses the acidolysis of acetyloxybenzene sulfonate by amido acids, i.e., alkanoylamido-substituted alkanoic acids, at temperatures of about 200xc2x0 C. for several hours.
The synthesis of aryl alkanoate esters using an xe2x80x9cimpeller esterificationxe2x80x9d technique is known. For example, U.S. Pat. No. 2,082,890 discloses the simultaneous addition of acetic anhydride to a mixture of an alkanoic acid and a phenol to produce the aryl alkanoate ester. An improved impeller method for the synthesis of aryl alkanoate was introduced by E. J. Bourne and coworkers in Journal of the Chemical Society 1949, 2976-79. Bourne et al. disclose the use of TFAA in the synthesis of aryl alkanoate esters using milder conditions. The use of the TFAA impeller esterification method for the synthesis of a phenolsulfonate ester was first disclosed by Thomas C. Bruice et al. in the J.Am.Chem.Soc., 1968, 90, 1333-48. However, the Bruice et al. article does not report a reaction yield, uses an excess of both TFAA and carboxylic acid (relative to the SPS) and employs reaction conditions comparable to those reported by others for strong acid-catalyzed phenolsulfonate ester synthesis. Because TFAA is a relatively expensive chemical, economic considerations discourage its use in large-scale synthesis.
European Patent Publication EP 105,672 discloses the use of acetic anhydride (Ac2O) as an impeller in the preparation of phenolsulfonate alkanoate esters. According to the disclosure of EP 105,672, a C2-C3 anhydride first is added to a mixture of a phenolsulfonate and C6-C12 carboxylic acid and heated to 140-160xc2x0 C. and then the temperature is raised so that transesterification (acidolysis) occurs yielding the desired product. Although the reaction conditions are more severe, only one equivalent of nonanoic acid is used, Ac2O is inexpensive and the reaction yield is high.
The xe2x80x9cAc2O impellerxe2x80x9d method for the synthesis of phenolsulfonate esters also is disclosed in U.S. Pat. Nos. 4,735,740 and 5,650,527 and in German Patent Publication DE 3,824,901 A1. In each of the processes disclosed in these three patent documents, Ac2O is added to a carboxylic acid of low volatility in the presence of SPS, heated for an extended period of time at relatively high temperatures, e.g., 2-5 hours at temperatures greater than 120xc2x0 C., and acetic acid is removed at reduced pressure to drive the conversion of SPS to its carboxylic acid ester.
Three disadvantages are inherent to this approach: first, the use of acetic anhydride as an impeller results in a significant amount of acetate ester which must be converted by high temperature transesterification and removal of acetic acid; secondly, not only is transesterification by acidolysis slow, but equilibrium mixtures between acetate esters and other carboxylic esters does not greatly favor the other carboxylic esters and thus the concentration of acetic acid at equilibrium is relatively low; and finally, the low solubility of phenolsulfonate esters in the media employed in these inventions retards reaction progress and is a barrier to clean conversion to the desired products.
I have discovered that when TFA is used as a solvent, or as a major component of the solvent, the reactions of carboxylic acids with phenols impelled by TFAA proceed at unprecedented rates under milder conditions than previously reported. I also have discovered that the use of molar excesses of TFAA is not necessary. This last discovery substantially improves the viability of this method for industrial scale synthesis. The process of the present invention therefore comprise the preparation of aryl carboxylate esters by reacting a phenol with a carboxylic acid containing a total of up to about 24 carbon atoms in the presence of TFA and TFAA wherein the mole ratio of TFAA:phenol is about 3:1 to 0.1:1. Because of the solubility, especially of sodium and potassium phenolsulfonate esters, in TFA, the present invention is especially useful for the preparation of such phenolsulfonate esters. Highly concentrated solutions, e.g., 20-50 weight percent, of these esters can be produced. Such high solubility combined with efficient solvent separation due to the low boiling point of TFA make it uniquely suited to the manufacture of phenolsulfonate esters. Because TFAA impeller esterifications in TFA occur under very mild conditions, the common problem of color formation is not encountered. Likewise, these methods can be applied to the synthesis of a wide variety of phenol esters, including those containing functionality in the carboxylic acid, such as amido acids. For example, the reaction of sodium phenolsulfonate with N-nonanoyl-6-aminocaproic[6-(nonanoylamido)-hexanoic] acid in the presence of TFAA/TFA produces the benzenesulfonate ester in greater than 98% isolated yield in less than 30 minutes at temperatures of from 25 to 45xc2x0 C. The process of the present invention may be used with difunctional compounds such as dicarboxylic acids, e.g., adipic acid, and/or aromatic diols, e.g., hydroquinone and resorcinol, that are of interest in the preparation of polymeric materials. In most cases, the isolation and purification of products is reduced to simply removing the TFA (b.p. =72xc2x0 C.) and any excess TFAA (b.p.=40xc2x0 C.) by evaporation. As mentioned, certain of the carboxylate esters which may be prepared by the present process are useful as bleach activators while others, especially arylene dicarboxylates and diaryl dicarboxylates, are useful in the preparation of polymers. The carboxylate esters also are useful as esterification agents for producing a variety esters.
The process of provided by the present invention is a process for the preparation of an aryl carboxylate ester which comprises contacting or reacting a phenol with a carboxylic acid in the presence of TFA and TFAA wherein the mole ratio of TFAA:phenol is about 3:1 to 0.1:1. The phenol reactant may be unsubstituted phenol or naphthol or a hydroxybenzene or hydroxynapthalene compound which may be substituted with a variety of substituents, usually not more than two, such as alkyl of up to about 12 carbon atoms, alkoxy containing up to about 12 carbon atoms, alkanoyl of up to about 12 carbon atoms, halogen such as chloro and bromo, sulfo, an alkali metal salt of sulfo such as sodium and potassium sulfo salts, alkanoylamido containing up to about 20 carbon atoms, nitro, formyl, cyano, alkoxycarbonyl containing 2 to 12 carbon atoms, carbamoyl and the like. The phenol reactant also may be substituted with a second hydroxy group, i.e., 1,2-, 1,3- and 1,4-benzenediols which result in the formation of arylene bis(alkanoate) esters. Additional aromatic diols which may be used include 1,4-naphthalenediol, 4,4xe2x80x2-sulfonyldiphenol and 4,4xe2x80x2-biphenol. The phenol reactant preferably is unsubstituted phenol or an alkali phenolsulfonate, especially sodium phenolsulfonate. The unsubstituted aryl ester produced in accordance with the present invention may be sulfonated to prepare the alkali metal phenolsulfonate ester which are useful as bleach activators.
The carboxylic acid reactant may be an unsubstituted or substituted aliphatic, cycloaliphatic or aromatic carboxylic acid containing a total of up to about 24 carbon atoms. Mixtures of carboxylic acids may be used. The unsubstituted aliphatic acids, preferably unsubstituted alkanoic acids, typically contain 4 to 20, preferably about 6 to 16, carbon atoms. The alkanoic acid, i.e., a saturated, aliphatic carboxylic acid, may be substituted with one or more, typically one, substituent selected from alkoxy containing up to about 12 carbon atoms, halogen such as chloro, alkanoylamido containing up to about 12 carbon atoms, aryl such as phenyl and phenyl substituted with alkyl, alkoxy and/or halogen. The alkanoic acid may be substituted with a second carboxyl group, e.g., adipic acid, azelaic acid and the like, which result in the formation of diaryl dialkanoate esters. The alkanoic acid reactant preferably is an unsubstituted alkanoic acid containing about 6 to 16 carbon atoms or an alkanoic acid containing about 6 to 16 carbon atoms which is substituted with an alkanoylamido group containing up to about 12 carbon atoms. The preferred alkanoic acid reactant includes mixtures containing two or more alkanoic acids containing about 6 to 16 carbon atoms, e.g., a mixture containing approximately 4% hexanoic, 54% octanoic, 39% decanoic and 1% dodecanoic acids.
The carboxylic acid reactant also may be selected from cycloaliphatic and aromatic, carbocyclic, carboxylic acids containing from about 6 to 24 carbon atoms such as cyclohexanecarboxylic acid, benzoic acid, and the naphthalenecarboxylic acids which may be unsubstituted or substituted with a wide variety, usually not more than two, of substituents such as alkyl of up to about 12 carbon atoms, alkoxy containing up to about 12 carbon atoms, alkanoyl of up to about 12 carbon atoms, halogen such as chloro and bromo, sulfo, an alkali metal salt of sulfo such as sodium and potassium sulfo salts, alkanoylamido containing up to about 12 carbon atoms, nitro, formyl, cyano, alkoxycarbonyl containing 2 to 12 carbon atoms, carbamoyl and the like. The cycloaliphatic and aromatic, carbocyclic, carboxylic acids also may be dicarboxylic acids such as 1,2-, 1,3- and 1,4-cyclohexanedicarboxylic acid, 1,2- , 1,3- and 1,4-benzenedicarboxylic acid and the many naphthalenedicar-boxylic acid isomers. The carboxylic acid and phenol may be used in carboxylic acid:phenol mole ratios in the range of about 2:1 to 0.5:1, preferably about 1.2:1 to 0.8:1.
TFAA is employed in an amount which gives a TFAA:phenol reactant ratio of about 3:1 to 0.1:1, preferably about 1.5:1 to 0.75:1. This is a feature of the present invention which distinguishes it from known processes which utilize significantly more TFAA. The amount of TFA solvent present initially and during the operation of the process of the present invention typically gives a TFA:phenol reactant mole ratio of at least 0.5:1 and preferably a TFA:phenol mole ratio in the range of about 2:1 to 20:1. Such mole ratios typically provide preferred amounts of TFA greater than 15 weight percent based on the weight of the phenol, carboxylic acid and TFAA present. The amount of TFA present preferably is in the range of about 30 to 80 weight percent based on the weight of the phenol, carboxylic acid and TFAA present. Other inert solvents may be used in conjunction with TFA but are not normally preferred. Examples of such solvents include halogenated hydrocarbons such as dichloromethane and dichlorobenzene; ethers such as diethylether and diglyme; aromatic hydrocarbons such as toluene; and polar aprotic solvents such as dimethylformamide, acetonitrile and sulfolane.
The solvent properties of TFA are unique and highly advantageous in the synthesis of phenolsulfonate esters. Exemplary data showing the solubility of 4-(nonanoyloxy)benzenesulfonate (NOBS) in a series of solvents is displayed in Table 1 wherein solubilities were measured at 23xc2x0 C. and expressed as grams of NOBS soluble in 100 grams of solution. It is notable that the only solvent in which NOBS is more soluble than water is TFA. I have found no better solvent for NOBS than TFA. This is surprising. First, examination of solvents 2-7 in Table 1 shows a reasonable correlation of NOBS solubility with solvent polarity. TFA is a non-polar solvent with a dielectric constant similar to that of acetic acid yet it dissolves more than ten times the amount of NOBS which is soluble in acetic acid at ambient temperature. It is likely that hydrogen bonding in TFA facilitates the solvation of sulfonate anions but it is notable, in contrast, that such effects are much weaker in acetic acid. Furthermore, while it is known in the art that dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) are relatively good solvents for benzenesulfonate esters they are much weaker solvents than TFA and because TFA has a much lower boiling point than these dipolar aprotic solvents it is the only good solvent for benzenesulfonate esters which can be readily stripped from product solutions and then purified and recycled with relative ease.
In Table 1, the dielectric constants and boiling points were taken from Christian Reichardt Solvent and Solvent Effects in Organic Chemistry; VCH: Weinheim, 1988; Trifluoroacetic Acid, by John B. Milne In Chemistry of Non-Aqueous Solvents, Vol 5B; J. J. Lagowski, Ed.; Academic Press: New York, 1978; pages 1-52 and G. Geisler and E. Manz Monat. Chem. 1969, 100, 1133-39. NMP is N-methylpyrrolidinone.
An important advantage of the present invention is that the novel esterification process may be carried out at relatively low temperature, e.g., temperatures in the range of about xe2x88x9210 to 80xc2x0 C., which results in improved selectivity to the desired product of higher quality due to the avoidance or minimization of the formation of color bodies. However, if desired, the process may be carried out over a broad range of temperatures, e.g., temperatures of about xe2x88x9250 to 250xc2x0 C. Pressure is not an important aspect of the present invention and, thus, the process may be carried out at pressures moderately above or below ambient temperature.
A further advantage of the present invention is its applicability not only to a wide variety of carboxylic acids but also to mixtures of carboxylic acids. Because fatty acids (especially from natural sources) are often obtained as mixtures of carboxylic acids, the ability to convert such a mixture to its corresponding mixture of benzenesulfonate esters offers a great advantage to a manufacturer that wishes to utilize such low cost feedstocks. For example, a mixture of alkanoic carboxylic acids known as C-810 is available from Procter and Gamble Chemicals. This mixture contains about 4% hexanoic, 54% octanoic, 39% decanoic and 1% dodecanoic acids. As described above, TFA is readily removed by evaporation under mild conditions so purification of the products to high purity white powders is vastly simplified in contrast to processes described in the art which rely on crystallization and filtration to purify benzenesulfonate esters. Such methods, when applied to a mixture of benzenesulfonate esters, are complicated by variable rates of crystallization that exist for different benzenesulfonate ester products. The embodiment of the present invention is readily applied to the manufacture of products containing a mixture of benzenesulfonate esters.